The second link provided by jmyeet gives more details. Apparently they made a roughly flat surface by splitting the stone by cutting a line of holes into which

The second link provided by jmyeet gives more details. Apparently they made a roughly flat surface by splitting the stone by cutting a line of holes into which they fit dried wooden wedges. Then they soaked the wedges in water, causing them to swell, opening a crack along the line. Then they bashed this surface flatter by pounding dolerite balls on any imperfections. You can see traces of this process on the surfaces of the abandoned obelisk lying in Aswan.